The present invention relates to a mobile communication system and, in particular, to a terminal assisted handoff mobile communication system in which a handoff is requested from a mobile terminal to a host station.
In a mobile communication system, such as a cellular system, a base station radio device or a radio base station is arranged in each of several radio zones. Upon reaching a boundary between the radio zones, a terminal establishing communication with a base station radio device under the best propagation condition starts communication with a new base station radio device to maintain communication. This function is called handoff or hand-over.
In a code division multiple access (CDMA) cellular system, radio zones are identified using different spreading codes, respectively. Accordingly, it is not necessary to allocate different frequencies to the respective radio zones as required in a frequency division multiple access (FDMA) cellular system or a time division multiple access (TDMA) cellular system. That is, the same frequency can be allocated to base station radio devices arranged in the respective radio zones. Accordingly, in the CDMA cellular system, upon reaching a boundary between the radio zones, a terminal in communication with a certain base station radio device can also simultaneously communicate with one or more base station radio devices in the adjacent radio zones. Thus, a continuous or non-hit handoff can be achieved. A handoff of this type is called a soft handoff, wherein a terminal shifts from a current radio zone to an adjacent radio zone using the same frequency.
FIG. 6 schematically shows a structure of a conventional terminal assisted handoff CDMA cellular system. The cellular system shown includes a base station controller (host station) 12 and base station radio devices 10a, 10b connected thereto. For example, a terminal 11 communicating with the radio device 10b on a traffic channel of a spreading code b1.times.b2 receives a pilot channel of a spreading code a1 transmitted from the radio device 10a covering an adjacent radio zone and constantly measures a reception level thereof. If the measured reception level exceeds a preset threshold value, the terminal 11 determines that it has entered a soft handoff radio zone and transmits a soft handoff start-up request signal carrying identification data of the radio device 10a to the radio device 10b now in communication, using the traffic channel of the spreading code b1.times.b2.
Pilot channels transmitted from the base station radio devices 10a, 10b are spread by different spreading codes, such as a1 and b1, for identification of the radio zones. In the IS-95-A, the spreading codes a1 and b1 are PN (pseudorandom noise) code sequences, and spreading codes which differ per radio zone are allocated to the respective base station radio devices by time offset.
The base station controller 12 receives the start-up request signal from the terminal 11 via the base station radio device 10b now in communication. Then, the base station controller 12 transmits a soft handoff start-up signal to the base station radio device 10a represented by the identification data carried on the start-up request signal. Upon reception of the start-up signal, the radio device 10a starts communication with the terminal 11. Simultaneously, the base station controller 12 transmits a soft handoff command signal to the terminal 11 via the radio device 10b for notification of a spreading code a1.times.a2 to be used on a traffic channel of the radio device 10a. Upon reception of the soft handoff command signal, the terminal 11 recognizes the new spreading code a1.times.a2 and starts communication with the radio device 10a while communicating with the radio device 10b, so as to shift to the radio zone of the radio device 10a.
As described above, a terminal in communication measures a reception level of a pilot channel transmitted from a base station radio device in an adjacent radio zone and, if the reception level exceeds a threshold value, the terminal requests the start-up of a soft handoff to an adjacent base station controller via a base station radio device now in communication. The handoff system of this type is called a terminal assisted handoff system, which can reduce the throughput of a device provided in a base station radio device for receiving a signal from a terminal and the throughput of a base station controller.
FIG. 7 is a block diagram showing in detail a structure of the terminal in the conventional terminal assisted handoff CDMA cellular system. The terminal 11 comprises an antenna 16, a receiver 17, a transmitter 23, a rake receiving section 15, a decoder 19, a reception level measuring section 21, an encoder 22 and a controller 20.
A signal received through the antenna 16 is converted into a baseband signal at the receiver 17 and then inputted to the rake receiving section 15. The terminal 11 includes only one receiver 17 for reduction in weight and size. However, if a frequency of an adjacent radio zone is the same as that of a radio zone now in communication, the terminal 11, while keeping the communication, can receive a pilot channel of the adjacent radio zone and measure a reception level thereof.
The rake receiving section 15 comprises a plurality of correlators 13.sub.1 to 13.sub.n (n is an integer not less than 2). In the correlators 13.sub.1 to 13.sub.n, spreading codes of mutually different delay times are set for efficient reception of signals coming through various propagation paths as in multipath fading peculiar to the cellular system. Specifically, in the correlators 13.sub.1 to 13.sub.n-1, spreading codes used in a base station radio device now in communication are set for receiving signals from that base station radio device. On the other hand, in the correlator 13.sub.n, spreading codes used for pilot channels of base station radio devices covering adjacent radio zones are set.
The baseband signal inputted to the rake receiving section 15 is subjected to correlative demodulation at each of the correlators 13.sub.1 to 13.sub.n-1, then combined and decoded at the decoder 19, and sent to the controller 20. On the other hand, in the correlator 13.sub.n, the pilot channels of the adjacent radio zones are subjected to correlative demodulation. The reception level measuring section 21 derives reception levels of the pilot channels of the adjacent radio zones based on results of the correlative demodulation at the correlator 13.sub.n and feeds them to the controller 20.
The controller 20 stores a preset threshold value for the start-up of a soft handoff and compares the reception levels of the pilot channels of the adjacent radio zones received from the reception level measuring section 21 with the preset threshold value. If the reception level exceeds the threshold value, the controller 20 produces a soft handoff start-up request signal carrying identification data of the base station radio device transmitting that pilot channel and transmits it to the base station radio device now in communication. The base station controller receives the start-up request signal via the base station radio device now in communication and transmits a soft handoff start-up signal to the base station radio device represented by the identification data. Then, the soft handoff is carried out and communication between the terminal and the base station radio device covering the adjacent radio zone is started.
FIG. 8 schematically shows a radio zone structure in the conventional terminal assisted handoff CDMA cellular system. The shown radio zone structure includes a plurality of adjacent radio zones A, B and C. The radio zone A is provided with a cell 24.sub.1 using a frequency f.sub.1, the radio zone B is provided with a cell 251 using the frequency f.sub.1 and a cell 25.sub.m using a frequency f.sub.m, and the radio zone C is provided with a cell 26.sub.1 using the frequency f.sub.1 and a cell 26.sub.m using the frequency f.sub.m The cells 24.sub.1, 25.sub.1 and 26.sub.1 of the radio zones A-C are adjacent to each other, and the cells 25.sub.m and 26.sub.m of the radio zones B and C are adjacent to each other. The cell using the frequency f.sub.m exists in each of the radio zones B and C, while it does not exist in the radio zone A. This is caused by difference in scale between the respective base station radio devices.
In the foregoing radio zone structure, the handoff is carried out without any problem when, for example, a terminal 11.sub.1 starting communication with the cell 25.sub.m shifts to the cell 26.sub.m of the radio zone C, or a terminal 11.sub.2 starting communication with the cell 25.sub.1 shifts to the cell 24.sub.1 of the radio zone A or to the cell 26.sub.1 of the radio zone C.
As described before, since the terminal 11 incorporates only one receiver 17, it can not convert a different frequency signal into a baseband signal during communication. Accordingly, if a frequency of a pilot channel of an adjacent radio zone for which the terminal 11 is headed (hereinafter, this radio zone will also be referred to as "headed radio zone") differs from a frequency now in use, the terminal 11 can not receive the pilot channel of the headed radio zone. For example, it is assumed that the terminal 11.sub.1 starting communication with the cell 25.sub.m at the frequency f.sub.m shifts to the radio zone A. In this case, since there is no cell using the same frequency f.sub.m in the headed radio zone A, the terminal 11.sub.1 can not receive a pilot channel of the headed radio zone A. Specifically, since the terminal 11.sub.1 can not receive a pilot channel of the cell 24.sub.1 at the frequency f.sub.1 in this case, it can not even carry out a handoff while changing the frequency. Accordingly, in the terminal assisted handoff system of the IS-95-A, when the terminal 11 moves to a radio zone where a frequency now in use can not be used, the handoff can not be achieved to cause disconnection in communication. The foregoing handoff to be carried out while changing a frequency is called a hard handoff.